Rational design of a thermalresponsive-polymer-switchable FRET system for enhancing the temperature sensitivity of upconversion nanophosphors.

Here we propose a thermoresponsive polymer PNIPAM modulated fluorescence resonance energy transfer (FRET) system to enhance the temperature sensitivity of upconversion nanophosphors (UCNPs). By utilizing red/near-infrared dual emitting NaLuF4:Mn(2+),Ln(3+) (Ln(3+) = Yb(3+), Er(3+), Tm(3+)) UCNPs as the energy donor and Au nanoparticles as the acceptor, the temperature resolution of the UCNPs is significantly increased from 3.1 °C to 0.9 °C in the physiological temperature range. Conjugating the UCNPs and acceptors into discrete nanocomposites in our samples facilitates reversible regulation of the emission intensity of UCNPs, which thus would extend their application range in biosensing, especially for probing the dynamic changes of local micro-environments in biological tissues. As there are a broad variety of stimuli to which smart polymers can reversibly respond, our experiments are also extendable to various external conditions in local micro-environments, such as pH values, metal ions, glucose, and tissue-specific enzymes.